Studies are being conducted on the extent to which direct noncovalent interactions among ribosomal proteins are important in maintaining the structure of ribosomes. Screening experiments involving gel filtration and cross-linking studies on mixtures of ribosomal proteins in solution are being used to identify those pairs or groups of proteins which form complexes with each other in the absence of ribosomal RNA. Such proteins will be purified for further characterization of putative complexes. The possibility of complex formation among the three proteins which bind to 5S RNA is being investigated. A second line of investigation is the development of chemical techniques for covalently cross-linking ribosomal proteins of ribosomal RNA. The effects of glutaraldehyde are currently being investigated, and other potential cross-linking reagents will be tested.